1. Field of the Invention
The present invention relates to an optical recording medium of an air-sandwich sealed type, which comprises a pair of concentrically-positioned disk-shaped polycarbonate substrates with a recording layer on a single surface of at least one of these substrates, which is constructed in such a configuration that the recording layer is provided on the inside, either with or without an inner peripheral spacer and an outer peripheral spacer, using a particular adhesive.
2. Discussion of Background
In order to ensure that there is no effect on the enclosed recording material in a disk-shaped information recording medium, adhesives used for sealing this kind of information recording medium are required to satisfy the following conditions: (1) no byproducts are formed during the curing process of the adhesive, and (2) no solvents are contained in the adhesive. For these reasons, epoxy-type adhesives are usually used. However, problems arise inasmuch as a plastic substrate of the recording medium undergoes deformation when epoxy-type adhesives are cured under application of heat thereto, and the curing takes considerable time. Cold-setting epoxy adhesives which are curable at room temperature have also been proposed, but the problem of a long curing period still exists with this type of adhesive.
Accordingly, considerable attention has been focused on ultraviolet-curable adhesives in recent years. These adhesives can satisfy the above conditions (1) and (2). However, in the case where a polycarbonate is used for a substrate of the disk-shaped recording medium, it is necessary to use the adhesive comprising a low-molecular-weight monomer which can dissolve polycarbonate to a certain degree to obtain adequate adhesion to the polycarbonate. Because this type of monomer is generally highly volatile, there is the problem, especially in the so-called air-sandwich type disk-shaped recording medium, that the low-molecular-weight monomer component volatilizes in the enclosed portion within a short time before the curing of the adhesive is completed. This has an adverse effect on a recording layer of the recording medium.
The inventors of the present invention have intensely investigated a large number of adhesives for sealing optical recording media and have discovered that the compound having the following general formula (I) shows superior adhesion to the polycarbonate and has low volatility. ##STR1## wherein k is an integer of 1 to 3.
Therefore, the inventors of the present invention have already proposed in Japanese Laid-Open Patent Application 63-20034 an adhesive containing the aforementioned compound as the adhesive for sealing the disk-shaped optical recording medium. In the above application, it was also stated that the adhesive strength of the adhesive can be effectively improved when the compound represented by the general formula (I) is used in combination with a so-called monofunctional monomer, which is an acrylate monomer having one acryloyl group (CH.sub.2 .dbd.CHCOO--) in one molecule, or a multifunctional monomer having two or more acryloyl groups in one molecule.
However, the above monofunctional or multifunctional monomers, which are mixed with the compound of formula (I), have comparatively low viscosity. When the above monomers are mixed with the compound (I) to prepare an adhesive, the viscosity thereof is too low, for example, about 100 cps, so that drawbacks remain, such as poor operability in using this material as a sealing adhesive.